1. Field of the Invention
This invention pertains to a process for removing dissolved oxygen from liquids.
2. Background of the Invention
Processes for the removal of dissolved oxygen from liquids have applications in many diverse fields. In a number of industries, including those of beverage making, electronics, aerospace, deep well injection, and power generation, water is used in great quantities and the presence of unsatisfactory levels of dissolved oxygen can present numerous problems, including inferior product quality and damaged process equipment. As one example, dissolved oxygen contained in hot water that is circulated through power generating equipment and the like is a major cause of corrosion. Because of the enormous costs of replacing corroded power generating equipment parts, unsatisfactory levels of dissolved oxygen cannot be tolerated.
In prior art deoxygenation processes, hydrazine has been used as a strong reducing agent to prevent corrosion and other problems associated with oxygenated water. A small amount of hydrazine is added to the water to react with the dissolved oxygen to form nitrogen and water. In the field of power generation, a small amount of hydrazine is also provided in the deoxygenated water as it circulates in the power generating equipment. The circulating hydrazine and water mixture is said to cause the formation of magnetite on metal surfaces of the equipment by the reaction of hydrazine with iron, and the magnetite in turn helps protect against corrosion. Hydrazine also reduces red iron oxide deposits that typically form in power generating equipment and impede heat transfer and cause tubes to rupture. The product of this reduction reaction is magnetite which will settle to the bottom of a water stream and which can thereby be effectively and economically removed from generating equipment.
It is known that the reaction of hydrazine with dissolved oxygen can be catalyzed by passing the hydrazine and water mixture through a bed of activated carbon. Such a catalyzed deoxygenation process is described in F. R. Houghton, et al, "The Use of Activated Carbon With Hydrazine in the Treatment of Boiler Feedwater", International Water Conference, Bournemouth, England (1957) on pages 54-58, wherein boiler feedwater containing between 5 and 7 parts per million of dissolved oxygen is dosed with hydrazine in an amount of from 30 to 70% over the stoichiometric amount necessary to react with the dissolved oxygen. The dosed water is subsequently passed through a bed of activated carbon and then fed directly into a boiler.
Despite the advantages that one skilled in the art might expect from the catalyzation of a deoxygenation reaction, the prior art teachings of activated carbon catalysis of hydrazine deoxygenation have been almost completely ignored by the art due to a number of disadvantages inherent in the prior art processes.
A first disadvantage of the catalyzed deoxygenation processes of the prior art arises from the introduction of impurities, such as unreacted hydrazine and carbon contaminants, into the deoxygenated liquid. In the process of removing dissolved oxygen, the prior art systems leave levels of unreacted hydrazine that cannot be tolerated when a liquid such as water is used in certain sophisticated equipment or for the production of refined products. Among other unsatisfactory effects, unreacted hydrazine can raise the conductivity and the pH of the deoxygenated water to unsatisfactory levels. The processes of the prior art also introduce carbon contaminants into the deoxygenated liquid and the presence of these contaminants likewise is intolerable when a deoxygenated liquid of high purity is required. In the field of power generation, for example, such impurities render the prior art processes useless in high pressure equipment and, as a result, the significant benefits of carbon-catalyzed water deoxygenation have been unavailable to the art.
Even when a certain amount of unreacted hydrazine is desirable in the deoxygenated water for the purpose of inhibiting corrosion during circulation in power generating apparatus, the prior art processes are inadequate in that no provisions are made for effectively adjusting the amount of unreacted hydrazine remaining after deoxygenation in order to provide the optimum amount of hydrazine in the circulating water. Thus, in selecting an optimum amount of hydrazine to be reacted during the deoxygenation stage, a residual amount of unreacted hydrazine can result which will be either higher or lower than the optimum amount for the circulating stage. If the amount during the circulating stage is too low, the anticorrosive effects of the hydrazine are lost, and if the amount during the circulating stage is too high, the pH and conductivity of the circulating water can be raised to unacceptable levels. Some prior art processes have attempted to remedy this defect when the residual amount of unreacted hydrazine is too low by simply providing a means for adding hydrazine before the circulating stage. However, this type of arrangement fails to remedy the defect when the amount is too high and, further, completely fails to allow the flexibility needed to enable one to use a different anti-corrosive agent than hydrazine in the circulating stage.
Still another shortcoming of the prior art is the failure to fully appreciate and address the hazards associated with hydrazine. Not only does hydrazine present a severe explosion hazard, especially when exposed to heat or oxidizing materials, but it is also highly toxic by ingestion, inhalation, and even skin absorption, and is a strong irritant to skin and eyes. Unconfirmed reports link hydrazine to cancer. In the prior art, no special precautions have been suggested for the handling of hydrazine used in deoxygenation processes nor for the handling of material beds that contain hydrazine.
Thus, there is a long-felt and unresolved need for an improved carbon-catalyzed deoxygenation process employing hydrazine that can be used commercially in a variety of applications. Ideally, the process would secure for the art all of the advantages that catalyzation of a reaction normally provides without prohibiting its use due to all of the incumbent disadvantages discussed hereinabove. To the fulfillment of this need and to other objectives that will become apparent from the following, the present invention is directed.